<!--
 * @Author: pengmingming
 * @Date: 2020-12-25 09:24:00
 * @LastEditors: pengmingming
 * @LastEditTime: 2020-12-25 09:37:04
-->
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .callout {
            font-size: 18px;
            position: relative;
            display: inline-block;
            border-radius: 4px;
            padding: 10px;
            background: lightsteelblue;
            color: black;
            border: 1px solid gray;
        }
        .callout::before {
            content: '';
            position: absolute;
            border: inherit;
            padding: 0.25em;
            border-right: 0;
            border-bottom: 0;
            transform: rotate(45deg);
            top: -0.32em;
            left: 0.9em;
            background: inherit;
        }
    </style>
</head>
<body>
    <div class="callout">
        2020年12月25日，css揭秘。
    </div>
</body>
</html>